1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device used as a display screen of a portable personal computer or the like.
2. Description of the Related Art
Conventionally, a color liquid crystal display (LCD) panel has been known as a display device of a portable personal computer or the like.
The color LCD panel is manufactured by, for example, sandwiching a liquid crystal between a TFT (thin film transistor) array substrate and a color filter array substrate. A color LCD module produced by attaching peripheral members such as a driver LSI and a light unit to the panel for modularization is integrated within a main body cover of the portable personal computer and thereby used as a display screen.
The portable personal computer in which the color LCD panel is mounted can be freely carried and used because of its portability. Thus, even when a vibration is imparted to the computer during carriage of the computer or the like, the panel is required to maintain its display quality. Accordingly, a vibration is imparted to the color LCD panel as a quality test to secure the reliability of LCD panel before shipment.
FIGS. 1A and 1B show the configuration of a conventional color LCD panel and FIG. 1A is a sectional view thereof, and FIG. 1B is a sectional view thereof for explaining an abnormal alignment. As shown in FIG. 1A, the conventional color LCD panel 1 includes: a TFT array substrate 2; a color filter array substrate 3; and a liquid crystal layer 4 sandwiched between both the substrates 2 and 3.
Alignment films 5 are respectively formed on surfaces of the TFT array substrate 2 and the color filter array substrate 3, the surfaces being disposed opposite each other. The liquid crystal layer 4 surrounded by a sealant 106 that covers a sealing spacer 6 is formed between the TFT array substrate 2 and the color filter array substrate 3. A gap formed between both substrates 2 and 3 (cell gap) and representing a thickness of the liquid crystal layer 4 is adjusted by a plurality of spacers 7 located therebetween.
The spacers 7 consist of particles having a spherical shape, a columnar shape or the like. Generally, a LCD panel employs spherical shaped particles having diameters characterized by a standard deviation of 3% to 5% of average particle size. Thus, a pressure applied to the spacers varies depending on different diameters of the respective spacers 7 while causing a variation in an amount of deformation of the respective spacers 7.
In other words, after completion of assembly of cell, the spacers 7 are in a compressed state and the spacer 7 having a small diameter is rarely deformed, and the spacer 107 having a large diameter is deformed to a large extent (FIG. 1B).
In this state, when a vibration is imparted to the color LCD panel 1, the small spacer 7 moves within the panel (see arrows in FIG. 1B). Even when the spacer 7 having an average diameter and deformed to a small extent moves, it moves causing small friction with the alignment films and therefore, never causes change of alignment in the alignment films.
However, particularly in the case where the spacer 107 having a large diameter moves within a large panel, imparting large pressure to the TFT array substrate 2 and the color filter array substrate 3 deforms the panel to thereby move the spacers. Thus, the friction between the moved spacers and the surfaces of both substrates 2 and 3 damages the alignment films 5.
As a result, the alignment films 5 is again aligned along the direction in which the spacer moves, thereby producing a bright point as a lighting point on a display screen. The bright point occurs more easily in proportion to the size of a screen. It would appear that in proportion to the size of a screen, the deformation of the panel is enhanced to partially impart large force to the panel. Hereinafter, the bright point thus generated is called a vibration bright point.
FIG. 2 is an explanatory view showing a bright point produced by the abnormal alignment observed in the color LCD panel shown in FIG. 1B. As shown in FIG. 2, a pixel is defined by drain wirings 108 and gate wirings 208 and driven by a TFT 9. When the spacer 107 having a large particle size moves within such a pixel to cause an abnormal alignment that changes the alignment of the alignment films 5, a region to be displayed normally becomes an abnormal region corresponding to a bright point.
Thus, when the abnormal region corresponding to the bright point is visually recognized on an image, a display contrast of the image is lowered (see JP 09-127515 A).
In order to prevent a vibration bright point from occurrence in a vibration test that is conducted before shipment of an LCD panel, it is necessary to make a particle size distribution of particles within a predetermined range of values so as not to include a spacer having a large particle size.
With respect to the particle size distribution of particles, in a period during which the size of a screen of portable personal computer or the like has been small as observed when a conventional portable personal computer was prevalently used, an LCD panel is also small in size and deformation of the LCD panel due to vibration itself is small, whereby a bright point rarely occurs. Even when a standard deviation of a particle size distribution of particles is of 3% to 5% of average particle size, vibration imparted to the LCD panel is suppressed small.
Recently, the portable personal computer having a large screen size (for example, 14.1-inch type in a notebook personal computer) has been prevalently used making the size of an LCD panel enlarged and therefore, a particle size distribution of particles observed in the conventional LCD panel becomes impermissible in terms of prevention of failure due to occurrence of bright point, thereby causing necessity of making a particle size distribution of particles not greater than 3% of average particle size.
In other words, since the LCD panel enlarged in size has a structure in which an interval between points to support the panel increases, the LCD panel is deformed to a larger extent when external force is imparted thereto, so that the LCD panel partially receives large force (not less than 10 times the external force in some cases).
Such a phenomenon largely affects a LCD panel when the panel is assembled by a front mounting method as one of assembly methods for an LCD panel. That is, when using the front mounting method, as compared with other mounting methods, an interval between points to support the panel is large and the panel itself is not secured to a case by screws, increasing the allowance between the case and the LCD panel. Accordingly, when force is locally imparted to the panel, the spacers within the LCD panel are forcibly moved, causing a bright point due to the realignment of alignment films.
Also, up to now, although a generally used resin spacer has been formed by a suspension polymerization method (the method is widely used in a polymer synthesis technique, in which a liquid organic solvent is dropped into a solvent such as water and after the organic solvent becomes a colloid state as milk, the resultant solvent is heated to obtain particles having the same size as that of the organic solvent during operation for dropping the organic solvent. The drawbacks of the method are that a variation in an amount of organic solvent becomes large during operation for dropping the organic solvent, so that the variation in sizes of the obtained particles becomes large), when a spacer is formed by the suspension polymerization method, a size distribution of particles becomes widely spread and therefore, an operation for classifying particles into some groups of particles based on a target particle size has been necessary. (The classifying operation is performed as follows: mix a powder into a solvent (such as water or an organic solvent, which does not allow the powder to dissolve therein); and stir the solvent; and then, leave the solvent as it is to obtain only particles having a target particle size while removing particles having a size different from the target particle size, in which operation large sized particles are precipitated in a relatively short time and small sized particles are precipitated in a relatively long time.)
To complete the operation for classifying particles, it takes about two weeks in the case where a standard deviation of a particle size distribution is 5% or less and about one month in the case where the standard deviation of the particle size distribution is 4% or less, and about two months in the case where the standard deviation of the particle size distribution is 3% or less, resulting in difficulty in making the standard deviation of the particle size distribution not greater than 3% in terms of prevention of time-consuming process and high manufacturing cost.
In contrast, a seed polymerization method of making a seed particle (microparticle as a nucleus in polymerization reaction) grow in a snowballing manner to produce a particle has recently been developed. The advantage of the method is that particles each having nearly the same particle size are advantageously extracted by controlling conditions (temperature, time, and the like). When employing the seed polymerization method, a variation in sizes of the extracted particles itself is very small (standard deviation "sgr"= about 2.6% to 3%) and therefore, the occurrence of a bright point can be suppressed. However, when two seed particles are bonded to each other forming a particle having an abnormal size, the particles having such an abnormal particle size are inevitably and unfavorably contained in the extracted particles.
As described above, in order to address the enlargement of the LCD panel, it is necessary to make the standard deviation of the particle size distribution not greater than 3% of average particle size. However, it is substantially impossible and impractical to meet the above-mentioned requirement by using the operation for classifying particles. Also when employing the seed polymerization method in which a variation in particle sizes is very small (standard deviation "sgr"= about 2.6% to 3%), since two seed particles are bonded to each other forming a particle having an abnormal size, the particles having such an abnormal particle size are inevitably and unfavorably contained in the extracted particles.
An object of the present invention is to provide a color liquid crystal display device in which a group of spacer particles are produced by a seed polymerization method so that spacers are made to have a standard deviation of particle size distribution of not greater than 3% of average particle size while excluding abnormal group of particles therefrom to prevent occurrence of vibration bright points at the time of vibration test even when the LCD panel is enlarged.
In order to attain the above-mentioned object, a liquid crystal display device of the present invention is constructed as follows. That is, the liquid crystal display device includes a liquid crystal layer, two substrates disposed opposite each other while sandwiching the liquid crystal layer therebetween, a group of spacer particles dispersed in the liquid crystal layer, in which the group of spacer particles have a standard deviation of particle size distribution of not greater than 3% of an average particle size and exclude particles having a particle size not less than 1.14 times the average particle size therefrom.
As described above, according to the present invention, in a liquid crystal display device in which a liquid crystal layer is sandwiched between two substrates disposed opposite each other through a plurality of spacers, the individual spacers is constructed such that spacer materials having a standard deviation of particle size distribution of not greater than 3% of average particle size are produced by a seed polymerization method and classified into groups of particles to exclude abnormal group of particles from the spacer materials. Accordingly, allowing spacers to have a standard deviation of particle size distribution of not greater than 3% of average particle size while excluding abnormal group of particles from the spacers makes it possible to prevent occurrence of vibration bright points at the time of vibration test even when the LCD panel is enlarged.